Gear drive sprinklers have a larger riser diameter than spray heads and the pressure load on the riser against the sprinkler housing top is much greater so that having the riser come up and seat against the rubber is undesirable. In my U.S. Pat. No. RE 35,037 I disclosed a gear driven sprinkler where the riser bottoms against the retraction spring when it is loaded up by the irrigation system pressure in the body can of the sprinkler. Also shown is a long cylindrical riser seal portion that extends down to seal to the riser at its bottom circumference when the sprinkler is in the up extended position.
U.S. Pat. No. 4,316,579 is a good summary of riser seal prior art.
One problem with known devices is that the soft rubber seal portion tends to lose dimensional stability over time, and then effectiveness of the seal deteriorates. Also known seals have not been completely effective as a low pressure seal to prevent blow-by while the sprinkler is being pressurized.
Thus, a need exists for an improved seal construction for sprinkler risers.